VOLTAGE CONVERTER CAPABLE OF CORRECTLY ENABLING SECONDARY CONVERTING MODULE COUPLED TO OUTPUT PORT OF VOLTAGE CONVERTER BY UTILIZING SLOPE INDICATION SIGNAL HAVING DIFFERENT PULSE AMPLITUDES REPRESENTATIVE OF DIFFERENT SLOPE VALUES

Abstract

A method for converting an input voltage signal into an output voltage signal is disclosed. The method includes: providing a primary converting module and coupling the primary converting module to an input port of the voltage converter; providing a secondary converting module having a second electronic induction device and a switch device, coupling the secondary converting module to an output port of the voltage converter, and utilizing the switch device to enable the secondary converting module; measuring a slope of an output at a detection end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values; and referencing the output at the detection end of the second electronic induction device, the slope indication signal, a first predetermined reference level, and a second predetermined reference level to generate a control signal for controlling an on/off status of the switch device.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a voltage converter, and more particularly, to a voltage converter capable of correctly enabling a converting module coupled to an output port of the voltage converter.

2. Description of the Prior Art

Please refer to FIG. 1. FIG. 1 is a diagram of an example of a prior art flyback voltage converter 100. As shown in FIG. 1, the voltage converter 100 comprises a primary converting module 105 and a secondary converting module 110. The primary converting module 105 has a first electronic induction device L₁ and a transistor Q₁, and is utilized for storing energy coming from the input port of the voltage converter 100 when the transistor Q₁ is turned on by a control signal S_c, and for transferring energy stored in the first electronic induction device L₁ to the secondary converting module 110 when the transistor Q₁ is turned off by the control signal S_c. The secondary converting module 110 has a second electronic induction device L₂ and a rectifying component 112 (implemented in this example by a diode D), and is utilized for transforming energy coming from the primary converting module 105 to generate an output voltage signal S_out. Usually, the voltage converter 100 works in a continuous conduction mode; however, it may work in a discontinuous conduction mode in certain situations, for example, in a situation where the voltage converter 100 only transfers 20 Watts to the output port of the voltage converter 100 even though power coming from the input port of the voltage converter 100 is higher than 20 Watts.

Please refer to FIG. 2 in conjunction with FIG. 1. FIG. 2 is a timing diagram illustrating operation of the prior art voltage converter 100 operating in the continuous conduction mode. As shown in FIG. 2, when the transistor Q₁ is turned on, the voltage level V₁ remains at a voltage level V_in (i.e. the voltage level of the input voltage signal S_in shown in FIG. 1) and the first electronic induction device L₁ is equivalently coupled to the ground voltage level where the voltage level V_ds is small and negligible. At this time, the current passing through the first electronic induction device L₁ (i.e. 11) increases slowly from zero up to a limit l_1max. Once the transistor Q₁ is turned off (i.e. at time t₁), the voltage level V₁ will be switched to another voltage level −(V_out+V_d)*N₁/N₂ immediately and the voltage level V_ds will become another voltage level V_in+(V_out+V_d)*N₁/N₂, wherein the value N₁/N₂ is a turns ratio between the first and second electronic induction devices L₁, L₂, and the voltage V_d is a conduction voltage of the diode D when forward-biased. At this time, the current I₁ will become zero and the current passing through the second electronic induction device L₂ (i.e. I₂) will increase immediately, then decrease slowly from a limit I_2max to zero to provide a voltage level of the output voltage signal S_out (i.e. V_out). Therefore, in continuous conduction mode the currents I₁, I₂ will not become zero at the same time; this does occur, however, in discontinuous conduction mode.

Please refer to FIG. 3. FIG. 3 is a timing diagram illustrating operation of the prior art voltage converter 100 operating in discontinuous conduction mode. As shown in FIG. 3, a difference of the current I₂ compared to the current in FIG. 2 is that the current I₂ shown in FIG. 3 reaches zero at time t₂ and remains at zero during a time period t₂˜T. The voltage level V_ds will then oscillate during the time period t₂˜T due to the parasitic capacitance/inductance within the voltage converter 100 itself. The diode D can still operate correctly even though the operation of the diode D is controlled by the voltage drop across itself and the oscillation of the voltage level V_ds also causes the voltage drop across the diode D to oscillate. An only disadvantage is that a power loss is introduced due to the voltage drop across the diode D. In general, a switch device implemented by a transistor is utilized for replacing the operation and function of the diode D in the secondary converting module 110. However, another power loss is introduced since the transistor may operate erroneously.

In general, there are two prior art schemes capable of avoiding the power loss caused by the above-mentioned transistor. One of the prior art schemes detects the current passing through the second electronic induction device L₂ (i.e. I₂) to control the transistor for avoiding the power loss. This scheme has to add a current sensing resistor or a current sensing transformer in the voltage converter 100, however. Considering total cost of the voltage converter 100, adding a current sensing resistor or current sensing transformer is not desirable.

Another prior art scheme capable of avoiding the power loss directly detects the waveform of the voltage level between the second electronic induction device L₂. Please refer to FIG. 4. FIG. 4 is a diagram of an example of a prior art flyback voltage converter 400 capable of avoiding the power loss. As shown in FIG. 4, the voltage converter 400 comprises a primary converting module 405, a secondary converting module 410, and a synchronous rectification controller 415. The secondary converting module 410 comprises a second electronic induction device L₂ and a switch device (in this example, it is implemented by a transistor Q₂). The synchronous rectification controller 415 is utilized for generating a control signal V_c to control an on/off status of the transistor Q₂ in the secondary converting module 410 by directly detecting the waveform of the voltage level at the node Z shown in FIG. 4.

Please refer to FIG. 5. FIG. 5 is a timing diagram illustrating operation of the prior art voltage converter 400 operating in the discontinuous conduction mode. As shown in FIG. 5, for example, when the control signal S_c remains at a high logic level during a time period t₁˜t₂, the transistor Q₁ in the primary converting module 405 shown in FIG. 4 is turned on. The node Z shown in FIG. 4 is regarded as a floating node since the status of the transistor Q₂ remains off. The voltage level at the node Z (i.e. V_z) then remains at a high voltage level during the time period t₁˜t₂. At time t₂, the second electronic induction device L₂ will be discharged for providing a stable voltage level of the output voltage signal S_out. The voltage level V_z will be decreased to zero Volts immediately since the transistor Q₁ will be turned off. Once the synchronous rectification controller 415 detects an immediately decreased voltage level V_z from the high voltage level to zero Volts, the transistor Q₂ is turned on by the synchronous rectification controller 415. Therefore, the second electronic induction device L₂ starts discharging for providing the stable voltage level of the output voltage signal S_out until time t₃, and the synchronous rectification controller 415 can control the status of the transistor Q₂ by detecting the immediately decreased voltage level V_z. In general, detecting the immediately decreased voltage level V_z is implemented by detecting a transition of the voltage level V_z from the high voltage level to a low voltage level based on a reference voltage level V_ref. The voltage level V_z may be unstable (i.e. the voltage level V_z may oscillate) during a time period t₃˜t₅, however, since the node Z is regarded as a floating node as mentioned above. It is possible for the synchronous rectification controller 415 to detect a transition of the unstable voltage level V_z and thus the transistor Q₂ is erroneously turned on by the synchronous rectification controller 415. For example, the transistor Q₂ is erroneously conducted in a time period t₄˜t₅ shown in FIG. 5. This will cause another problem for detecting the voltage level V_z to control the transistor Q₂. Therefore, controlling the transistor Q₂ only by detecting a transition of the voltage level V_z based on the reference voltage level V_ref has some disadvantages.

For solving the above-mentioned problem, the prior art scheme further generates a sensor pulse according to a plurality of predetermined levels V_A, V_B and a transition of the voltage level V_z from the high voltage level to the low voltage level. The sensor pulse and a reference pulse will be compared to determine whether the transition of the voltage level V_z is stable (i.e. the voltage level V_z does not oscillate at this time). Please refer to FIG. 6. FIG. 6 is a diagram illustrating two operation results of the prior art voltage converter 400 according to different transitions of the voltage level V_z and the reference pulse. As shown in FIG. 6, the left part of this diagram shows operation of the voltage converter 400 under the condition of a transition of an unstable voltage level V_z, while the right part of this diagram shows operation of the voltage converter 400 under the condition of a transition of a stable voltage level V_z′. Usually, a transition time of the unstable voltage level V_z is much longer than that of the stable voltage level V_z′. For example, the transition time of the unstable voltage level V_z from a high voltage level V_A to a low voltage level V_B may be up to 250 nanoseconds, yet the transition time of the stable voltage level V_z′ from the high voltage level V_A to the low voltage level V_B is only up to 50 nanoseconds. The synchronous rectification controller 415 can turn on the transistor Q₂ correctly by the sensor pulses T_AB, T_AB′ and the reference pulse T_ref. For example, the synchronous rectification controller 415 turns on the transistor Q₂ when it detects that the width of the sensor pulse T_AB′ is shorter than that of the reference pulse T_ref, and turns the transistor Q₂ off when detecting that the width of the sensor pulse T_AB is longer than that of the reference pulse T_ref. A primary defect of utilizing the above-mentioned sensor pulses is that the voltage level at the node Z in this voltage converter may be slightly different from that in another voltage converter. It is very possible that the sensor pulse T_AB′ does not correspond to a period when the voltage level V_z′ transits from the predetermined level V_A to the predetermined level V_B (i.e. the period that the voltage level V_z′ changes very sharply). If serious enough, a stable waveform of the voltage level at the node Z could be erroneously regarded as an unstable waveform. For instance, a stable waveform of a voltage level at the node Z in a different voltage converter could be regarded as an unstable waveform since the predetermined level V_A may be changed due to process drift. Another defect of the sensor pulses is that a starting timing of a pulse utilized for conducting the transistor Q2 of FIG. 4 is later than an ending timing of the reference pulse Tref. However, during the reference pulse Tref, the transistor Q2 may be turned on by its body diode voltage even though the control signal Vc generated from the synchronous rectification controller 415 is zero. Thus additional power dissipation may be introduced.

The present invention provides a new scheme for solving the above-mentioned problems in the discontinuous conduction mode without generation and comparison of the sensor pulse and the reference pulse.

SUMMARY OF THE INVENTION

One of the objectives of the present invention is to provide a voltage converter capable of correctly enabling a secondary converting module by measuring a slope of an output at the second end of a second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values, in order to solve the above-mentioned problem without requiring the reference pulse.

According to the claimed invention, a voltage converter for converting an input voltage signal into an output voltage signal is disclosed. The voltage converter comprises a primary converting module, a secondary converting module, and a switch controller. The primary converting module, having a first electronic induction device, is coupled to an input port of the voltage converter. The secondary converting module comprises a second electronic induction device and a switch device. The second electronic induction device is coupled to the first electronic induction device and it comprises a first end and a second end. The first end of the second electronic induction device is coupled to an output port of the voltage converter. The switch device is coupled to the second end of the second electronic induction device and utilized for enabling the secondary converting module when switched on. Additionally, the switch controller is coupled to the switch device and the second electronic induction device and it comprises a slope detecting circuit and a decision circuit. The slope detecting circuit is coupled to the second end of the second electronic induction device and utilized for measuring a slope of an output at the second end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values. The decision circuit is coupled to the slope detecting circuit and the switch device, and utilized for referencing the output at the second end of the second electronic induction device, the slope indication signal, a first predetermined reference level, and a second predetermined reference level to generate a control signal for controlling an on/off status of the switch device.

According to the claimed invention, a method for converting an input voltage signal into an output voltage signal is disclosed. The method comprises: providing a primary converting module having a first electronic induction device and coupling the primary converting module to an input port of the voltage converter; providing a secondary converting module comprising a second electronic induction device and a switch device, coupling a first end of the second electronic induction device to an output port of the voltage converter and a second end of the second electronic induction device to the switch device, and utilizing the switch device to enable the secondary converting module; measuring a slope of an output at the second end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values; and referencing the output at the second end of the second electronic induction device, the slope indication signal, a first predetermined reference level, and a second predetermined reference level to generate a control signal for controlling an on/off status of the switch device.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example of a prior art flyback voltage converter.

FIG. 2 is a timing diagram illustrating operation of the prior art voltage converter shown in FIG. 1 operating in continuous conduction mode.

FIG. 3 is a timing diagram illustrating operation of the prior art voltage converter shown in FIG. 1 operating in discontinuous conduction mode.

FIG. 4 is a diagram of an example of a prior art flyback voltage converter capable of avoiding power loss.

FIG. 5 is a timing diagram of an operation of the prior art voltage converter shown in FIG. 4 in discontinuous conduction mode.

FIG. 6 is a sketch diagram illustrating two operation results of the prior art voltage converter shown in FIG. 4 according to different transitions of the voltage level and the reference pulse.

FIG. 7 is a diagram of an embodiment of a voltage converter according to the present invention.

FIG. 8 is a timing diagram illustrating operation of the voltage converter shown in FIG. 7 while working in discontinuous conduction mode according to the present invention.

DETAILED DESCRIPTION

Please refer to FIG. 7. FIG. 7 is a diagram of an embodiment of a voltage converter 700 according to the present invention. As shown in FIG. 7, the voltage converter 700 comprises a primary converting module 705, a secondary converting module 710, and a switch controller 715. The primary converting module 705 comprises a first electronic induction device L₁ and a transistor Q₁, and the secondary converting module 710 comprises a second electronic induction device L₂ and a switch device 712. The operation and function of the primary converting module 705 and secondary converting module 710 are identical to that of the primary converting module 405 and secondary converting module 410 shown in FIG. 4, and therefore are not detailed for brevity. In this embodiment, the switch controller 715 comprises a slope detecting circuit 720 and a decision circuit 725. The slope detecting circuit 720 comprises an operational amplifier OP₁, a resistance unit 730, and a capacitor C. In this embodiment, the resistance unit 730 is implemented with a resistor R only. This is not a limitation of the present invention, however. The slope detecting circuit 720 is utilized for measuring slope values of an output at a detection end of the second electronic induction device L₂ (in this embodiment, the detection end of the second electronic induction device L₂ is mean to be the second end of the second electronic induction device L₂, i.e. the slope detecting circuit 720 measures the slope values of transitions of the voltage level V_z at the node Z) to generate a slope indication signal S₁ having different pulse amplitudes representative of different slope values. The decision circuit 725 comprises a plurality of comparators COMP₁, COMP₂, and a decision unit 732, where the decision unit 732 comprises a plurality of D-type Flip-Flops 735 and 740, an XNOR gate 745, and an AND gate 750. The decision circuit 725 is utilized for referencing the voltage level at the node Z (i.e. the voltage level V_z), the slope indication signal S₁ generated from the slope detecting circuit 720, a first predetermined reference level V₁, and a second predetermined reference level V₂ to generate a control signal V_c for controlling an on/off status of the transistor Q₂. Further description is detailed as below.

Please refer to FIG. 7 in conjunction with FIG. 8. FIG. 8 is a timing diagram illustrating operation of the voltage converter 700 while working in discontinuous conduction mode according to the present invention. As shown in FIG. 8, the waveform of the voltage level V_z shown in FIG. 8 is similar to that shown in FIG. 5. During a time period t₁˜t₃, the node Z is similarly regarded as a floating node since the discharging process of the second electronic induction device L₂ is finished and the charging process of the first electronic induction device L₁ is not yet started. The voltage level V_z therefore oscillates until the charging process of the first electronic induction device L₁ is started (i.e. the transistor Q₁ will be turned on at time t₃). After time t₃, the voltage level V_z continues to remain at a high voltage level until the transistor Q₁ is turned off at time t₄ and the voltage level V_z will be decreased immediately from the high voltage level to zero Volts at time t₄. In this embodiment, the slope detecting circuit 720 is implemented using a differential circuit well known to those skilled in the art; detailed operation is therefore not stated here for brevity. When the voltage level V_z remains at a stable voltage level, the slope indication signal S₁ also remains at a high voltage level V_bias shown in FIG. 8 since the non-inverting input of the operational amplifier OP₁ is coupled to the high voltage level V_bias. Once the voltage level V_z oscillates, the slope indication signal S₁ will not remain at the voltage level V_bias and will oscillate instead, as shown in FIG. 8. Please note that, for brevity, FIG. 8 only shows the voltage level V_z oscillating from a higher voltage level to a lower voltage level and the corresponding change of the slope indication signal S₁. As mentioned above, it is assumed that a transition time of an oscillated voltage level V_z is much longer than that of a stable voltage level V_z. That is to say, a transition slope of the oscillated voltage level V_z is not sharper than that of the stable voltage level V_z. For example, a transition slope of the voltage level V_z near time t₂ is not sharper than that of the voltage level V_z near time t₄. Therefore, the slope indication signal S₁ has different pulse amplitudes representative of different slope values. The second predetermined reference level V₂ is utilized for separating a pulse amplitude near time t₂ from another pulse amplitude near time t₄. Therefore, by comparing the second predetermined reference level V₂ and the slope indication signal S₁, the comparator COMP₂ will output a second reference control signal S₂′ having a high logic level when the slope indication signal S₁ has a pulse amplitude above the second predetermined reference level V₂, or output a second reference control signal S₂′ having a low logic level when the slope indication signal S₁ has a pulse amplitude below the second predetermined reference level V₂.

In addition, the comparator COMP₁ also compares the voltage level V_z and the first predetermined reference level V₁ to generate a first reference control signal S₁′ to determine the time during which the transistor Q₂ is conducted. The first reference control signal S₁′, however, will comprise a plurality of correct output pulses (e.g. an output pulse P₁) and a plurality of false output pulses (e.g. an output pulse P₂) due to the oscillation of the voltage level V_z. As mentioned above, the false output pulse (e.g. the output pulse P₂) will cause the transistor Q₂ to be conducted erroneously such that a great power loss is introduced. Therefore, the decision unit 732 is used for generating the control signal V_c according to the first and second reference control signals S₁′, S₂′. The operation of the combination of the D-type Flip-Flops 735 and 740, the XNOR gate 745, and the AND gate 750 can be readily understood; further description is not detailed for brevity.

In another embodiment, the slope detecting circuit 720 can also detect the transition slope of the voltage level V_z by detecting a transition of the voltage level V_z from a low logic level to a high logic level. In this situation, the voltage level V_bias will be coupled to a low voltage level. It is necessary for the voltage level V_z to be inversed and then transmitted into the input of the slope detecting circuit 720. This also obeys the spirit of the present invention, and falls in the scope of the present invention.

In summary, by using the slope detecting circuit 720 and the decision circuit 725, the switch controller 715 can control the on/off status of the transistor Q₂ correctly without requiring the generation of the above-mentioned reference pulse. The voltage converter 700 can therefore be enabled correctly for avoiding a great power loss. Being compared to the synchronous rectification controller 415, a primary advantage of the switch controller 715 is that the waveform of the voltage level at the node Z in the voltage converter 700 can be more correctly estimated as either a stable waveform or an unstable waveform. The reason follows: only when the voltage level at the node Z transits very sharply (e.g. time t₄), the generated slope indication signal S₁ shown in FIG. 8 also transits sharply and therefore becomes lower than the second predetermined reference level V₂. Accordingly, the estimation for the waveform of the voltage level at the node Z is not easily affected by changes of the above-mentioned predetermined levels V_A, V_B or other noise. Another advantage is that a starting timing of a pulse utilized for conducting the transistor Q₂ of FIG. 7 can be immediately outputted by the switch controller 715 (i.e. the waveform of the voltage level V_c shown in FIG. 8) when detecting that the voltage level V_z transits much sharply. The transistor Q₂ is turned on by the control signal V_c almost immediately at time t₄, and additional power dissipation is not introduced. Accordingly, a total power efficiency of the voltage converter 700 is improved.

Additionally, although the embodiment of the present invention is applied in a flyback voltage converter, it can also be applied in a forward voltage converter. Taking an example of the forward voltage converter, the detection end of the second electronic induction device is mean to be the first end of the second electronic induction device, and the slope detecting circuit measures slope values of an output at the first end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values. The decision circuit references the first end of the second electronic induction device, the slope indication signal generated from the slope detecting circuit, a first predetermined reference level, and a second predetermined reference level to control an on/off status of a switch device coupled to the second of the second electronic induction device. This also obeys the spirit of the present invention.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.

Claims

1. A voltage converter for converting an input voltage signal into an output voltage signal, comprising:

a primary converting module, comprising a first electronic induction device, coupled to an input port of the voltage converter;

a secondary converting module, comprising: a second electronic induction device, coupled to the first electronic induction device, the second electronic induction device comprising a first end and a second end, wherein the first end of the second electronic induction device is coupled to an output port of the voltage converter; and a switch device, coupled to the second end of the second electronic induction device, for enabling the secondary converting module when switched on; and

a switch controller, coupled to the switch device and the second electronic induction device, comprising: a slope detecting circuit, coupled to a detection end of the second electronic induction device, for measuring a slope of an output at the detection end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values; and a decision circuit, coupled to the slope detecting circuit and the switch device, for referencing the output at the detection end of the second electronic induction device, the slope indication signal, a first predetermined reference level, and a second predetermined reference level to generate a control signal for controlling an on/off status of the switch device; wherein the detection end of the second electronic induction device is selected from one of the first and second ends of the second electronic induction device.

2. The voltage converter of claim 1, where the primary and secondary converting modules comply with a flyback voltage converter configuration, and the detection end of the second electronic induction device is the second end of the second electronic induction device.

3. The voltage converter of claim 1, wherein the slope detecting circuit is a differential circuit, and the differential circuit comprises:

a capacitor, having a first end coupled to the detection end of the second electronic induction device;

a resistance unit, having a first end coupled to a second end of the capacitor; and

an operational amplifier, having a non-inverting input end for receiving a third predetermined reference level, an inverting input end coupled to the second end of the capacitor, and an output end coupled to a second end of the resistance unit;

wherein the slope indication signal is generated from the first end of the resistance unit.

4. The voltage converter of claim 3, wherein the decision circuit comprises:

a first comparator, coupled to the detection end of the second electronic induction device, for comparing the output at the detection end of the second electronic induction device and the first predetermined reference level to generate a first reference control signal;

a second comparator, coupled to the inverting end of the operational amplifier, for comparing the second predetermined reference level and the slope indication signal to generate a second reference control signal; and

a decision unit, coupled to the first comparator and the second comparator, for generating the control signal according to the first and second reference control signals.

5. The voltage converter of claim 4, wherein the decision unit comprises:

a first latch, having a clock input end coupled to the first reference control signal;

a second latch, having a clock input end coupled to the second reference control signal, a data input end coupled to an inverting data output end, and a non-inverting data output end coupled to a data input end of the first latch;

a XNOR gate, having two input ends coupled to the inverting data output end of the first latch and the non-inverting data output end of the second latch; and

an AND gate, having two input ends coupled to an output end of the XNOR gate and the first reference control signal, for generating the control signal to the switch device.

6. A method for converting an input voltage signal into an output voltage signal, comprising:

providing a primary converting module having a first electronic induction device and coupling the primary converting module to an input port of the voltage converter;

providing a secondary converting module comprising a second electronic induction device and a switch device, coupling one of a first end and a second end of the second electronic induction device to an output port of the voltage converter and coupling the second end of the second electronic induction device to the switch device, and utilizing the switch device to enable the secondary converting module when the switch device is switched on;

measuring a slope of an output at a detection end of the second electronic induction device to generate a slope indication signal having different pulse amplitudes representative of different slope values; and

referencing the output at the detection end of the second electronic induction device, the slope indication signal, a first predetermined reference level, and a second predetermined reference level to generate a control signal for controlling an on/off status of the switch device; wherein the detection end of the second electronic induction device is selected from one of the first and second ends of the second electronic induction device.

7. The method of claim 6, further comprising: coupling the primary and secondary converting modules utilizing a flyback voltage converter configuration; and selecting the second end of the second electronic induction device for the detection end of the second electronic induction device.

8. The method of claim 6, wherein the step of measuring the slope of the output at the detection end of the second electronic induction device comprises:

differentiating the output at the detection end of the second electronic induction device to generate the slope indication signal.

9. The method of claim 6, wherein the step of generating the control signal to control the on/off status of the switch device comprises:

comparing the output at the detection end of the second electronic induction device and the first predetermined reference level to generate a first reference control signal;

comparing the second predetermined reference level and the slope indication signal to generate a second reference control signal; and

generating the control signal according to the first and second reference control signals.